The present invention relates to electronic systems and, more particularly, to the field of electronic timing systems.
Over the years, various electronic timing systems, clocks, or clocking circuits for electronic systems have been developed. Clocks often use a crystal oscillator, e.g., a quartz-crystal resonator, for frequency stability. The very high stiffness and elasticity of piezoelectric quartz make it possible to produce resonators extending from approximately 1 KHz to 200 MHz. Clocks using a crystal oscillator, for example, have been developed which operate at low power and maintain good accuracy at low cost. The disadvantage of these clocks, however, is that they can maintain their timing accuracy only over a narrow temperature range. Outside this narrow temperature range, the frequency variation becomes quite large and the timing error increases considerably. Some of these timing inaccuracies, for example, can be attributed to the inadequate performance of the crystal oscillator.
The performance characteristics of a crystal oscillator, e.g., a quartz-crystal resonator, generally depend on both the particular cut and the mode of vibration. Each xe2x80x9ccut-modexe2x80x9d combination is considered as a separate piezoelectric element, and the more commonly used elements often are designated with letter symbols. The temperature coefficient of the frequency of the crystal varies with different cuts, i.e., with the crystal dimensions, and, generally, a parabolic frequency variation with temperature can be observed.
In order to improve the frequency accuracies of clocks, some clocks have also been developed which use a high precision crystal oscillator with a better temperature coefficient, such as a temperature compensated crystal oscillator (xe2x80x9cTCXOxe2x80x9d). The TCXO requires a temperature sensor and a more accurate crystal. These clocks, however, have the disadvantages of requiring considerably more power, size, and weight than the original simple clock. Also, these clocks are generally more expensive due to the complicated design and the high cost of the special crystal.
Another conventional approach for a clock is to use two crystals. Instead of using a high precision crystal oscillator and a temperature sensor to measure the temperature (e.g., a TXCO), a very temperature stable high frequency crystal or oscillator is used in this approach as a reference frequency. The high frequency crystal has good performance characteristics over the operating temperature range. In other words, the frequency change versus temperature variation is a relatively flat line instead of a parabolic curve. This high frequency crystal can be used to generate a reference frequency, for example, every 10 minutes. Meanwhile, another normal crystal, e.g., 32 KHz, of the clock also is always operating or running and requires only a low level of current. The normal crystal operates in a dual mode by turning one of the load capacitors on and off. This means that the crystal either has a fast frequency by about 75 parts per million (xe2x80x9cppmxe2x80x9d) or a slow frequency by 35 ppm. By comparing the 32 KHz frequency with the reference frequency every 10 minutes, the 32 KHz frequency can be adjusted automatically by selecting the dual mode operating time. Nevertheless, a clock using this approach is expensive and can be complex.
With the foregoing in mind, the present invention advantageously provides a cost effective temperature compensated real time clock which does not require an additional crystal or a microprocessor. The present invention also advantageously provides a real time clock and method that produce a timing signal which has been calibrated or compensated for various changes in temperature which may occur over time. The present invention further advantageously provides a simple, low power, and inexpensive real time clock and method for use in various systems.
More particularly, a temperature compensated clock is provided according to the present invention and preferably has waveform generating means for generating a waveform at a preselected frequency. Temperature monitoring means is advantageously responsive to a voltage input signal independent of temperature and a voltage input signal proportional to temperature for monitoring variations in temperature. The clock also has temperature compensating means responsive to the waveform generating means and the temperature monitoring means for compensating for frequency variations in the generated waveform due to temperature changes and thereby produce a temperature compensated output timing signal.
In a temperature compensated clock according to the present invention, the waveform generating means is preferably provided by an oscillator for generating an oscillating waveform signal at a preselected frequency and a frequency divider responsive to the oscillator for dividing the frequency of the oscillating waveform signal. The temperature monitoring means advantageously subtracts the input voltage signal proportional to temperature from the input voltage signal independent of temperature to thereby generate a difference signal. The input voltage signal proportional to temperature preferably is generated internal to the clock of the present invention. This difference signal preferably is converted to a digital format.
The temperature compensating means of the present invention preferably includes a programmable. scaling circuit, responsive to the generated waveform signal and the digital difference signal, for scaling the frequency of the generated waveform and thereby produce an accurate temperature compensated output timing signal. The programmable scaling circuit advantageously has pulse counting means for counting a predetermined total number of timing pulses. The pulse counting means preferably includes a pair of counters which separately count a predetermined portion of the total of number of timing pulses. At least one of the pair of counters is preferably programmable so that the accuracy of the desired scaled frequency output timing signal can be flexibly adjusted.
The programmable counter of the programmable scaling circuit preferably receives the digital difference signal periodically sampled from the temperature monitoring means and responsively counts the programmed number of pulses. The output of the pulse counting means provides a control signal for an input to scaling means for scaling the predetermined waveform frequency. The second counter of the pulse counting means, in turn, receives a divided and scaled output signal from a dividing circuit which is responsive to the scaling means. The second counter counts a number of pulses preferably proportional to the desired scaled frequency output timing signal.
By providing the temperature compensating means of the clock which includes a programmable scaling circuit according to the present invention, the clock can advantageously be flexibly adapted or designed for an accurate desired frequency output. Accordingly, the system designer can flexibly balance or make trade-offs between increased clock accuracy and costs or power usage. Also, by recognizing these flexible system constraints, a simplified and inexpensive real time clock, as well as methods of clocking systems, is provided according to the present invention.
The present invention also advantageously includes methods of clocking systems. A method of clocking systems preferably includes generating a waveform signal at a preselected frequency and monitoring temperature variations responsive to an input voltage signal independent of temperature and an input voltage signal proportional to temperature. The method also includes generating a difference signal representative of the difference between the input voltage signal independent of temperature and the input voltage signal proportional to temperature and scaling the frequency of the generated waveform responsive to the difference signal to thereby produce a temperature compensated output timing signal.
Another method of clocking systems includes monitoring an input voltage signal independent of temperature and an input voltage signal proportional to temperature for variations in temperature. Frequency variations in a generated waveform are compensated for in a system responsive to the monitored temperature variations to thereby produce a temperature compensated output timing signal.
By providing an internal temperature dependent voltage generating circuit and using a temperature independent voltage reference signal, a clock and methods of clocking systems of the present invention advantageously monitor temperature variations as a difference signal only at periodic times so to save power for the clock. This difference signal can advantageously be converted to a digital format so that the programmable scaling circuit can readily adjust for frequency variations due to temperature changes over time. The present invention also advantageously allows a low cost waveform generator, such as an inexpensive or low cost crystal, to be used as an input to the clock and yet produce a fairly accurate clock output signal the frequency of which does not vary greatly due to changes in temperature, i.e., compensates for frequency variations over temperature.